High-finesse limewater composition

ABSTRACT

A composition of milk of lime comprising particles of slaked lime suspended in an aqueous phase, characterized in that said particles of slaked lime have a particle size described by a particle size distribution profile that is narrow and monomodal and the method of production thereof.

The present invention relates to a composition of milk of limecomprising particles of slaked lime suspended in an aqueous phase and tothe manufacturing process thereof.

Suspensions of particles of slaked hydrated lime sometimes also calledmilk of lime, cream of lime or lime slurry are widely used industriallyas reagents in a multitude of applications, in particular in theneutralisation of waste water or acid effluent, pH adjustment and themineralisation of potable (drinking) water, neutralisation of chemicalreactions, for example, such as in the production of ethylene oxide orpropylene, as a source of calcium or for precipitation, in theproduction of vitamin C, of citric acid and precipitated calciumcarbonates (PCC), or also as adsorbent in the desulfurisation and theelimination of acid gases such as hydrochloric acid (HCL), in the fluegases.

Such suspensions of particles of slaked lime or milk of lime arecommonly obtained by the slaking of quicklime with a large excess ofwater or forming a suspension of slaked lime powder. The resultingparticles consist predominantly of calcium hydroxide.

Such slaked lime or calcium hydroxide may obviously contain impurities,that is, phases derived from SiO₂, Al₂O₃, Fe₂O₃, MnO, P₂O₅, K₂O and/orSO₃, broadly representing tens of grammes per kilogram. Nevertheless,the amount of these impurities, expressed in the form of oxides of theabove cited, does not exceed 5% by mass, preferably 3%, preferably 2% oreven 1% of the mass of the inorganic mineral filler according to theinvention. In particular, the slaked lime advantageously contains lessthan 1.0% by mass of Fe₂O₃, preferably less than 0.5% and preferablyless than 0.3%.

This slaked lime may also contain calcium oxide that would not have beenhydrated during slaking, just as it may contain calcium carbonate CaCO₃.The calcium carbonate may be from either the initial limestone fromwhich the slaked lime is derived according to the invention (unburnt),or from a reaction of partial carbonation of the slaked lime in contactwith air. The calcium oxide content in the slaked lime in the context ofthe present invention is generally less than 3% by mass, preferably lessthan 2% and in an advantageous manner less than 1% The calcium carbonatecontent is less than 10% by mass, preferably less than 6% and in anadvantageous manner less than 4%, and even more advantageously less than3%.

This slaked lime may also contain magnesium oxide MgO or derived phasesof types such as Mg(OH)₂ or MgCO₃, representing on the whole, tens ofgrammes per kilogramme. Nevertheless, the amount of these impurities,expressed in the form of MgO, advantageously does not exceed 5% by mass,preferably 3%, preferably 2% or even 1% of the weight of the inorganicmineral filler according to the invention.

The factors limiting the use of milk of lime in comparison with otherpotential reactants are generally linked to its viscosity and its rateof reaction or neutralisation in the reaction medium of the application.

Quite understandably, a high rate of reaction is desired as it enablesthe establishment of more rapid methods and therefore shorter processingperiods, and in an industrial context, would make it possible forsmaller probably less expensive equipment to be used for the reactionand/or industrial equipment having higher production capacity. Inaddition, in certain processes a predetermined minimum rate of reactionis often required in order to provide a product having the desiredqualitative characteristics.

Unfortunately, the viscosity is generally a limiting factor to allow theefficient use of fine milk of lime. Indeed, the suspension must bepumped so as to be fed from one point to the other and dosed, which isnot easy for highly viscous suspensions. In addition, highly viscoussuspensions generally diminish the quality of the dispersion, which hasan impact on the reaction rates observed for such applications since ittakes more time and energy to disperse the particles of slaked lime inthe reaction medium.

The viscosity and the rate of reaction of milk of lime are twoquantitative measures that are related to the size of particles.Particles having a reduced particle size generally have higher rates ofreaction, but also result in high viscosity of the suspension in whichthey are found, particularly when the solids content has to beincreased.

The rate of reaction of the particles of slaked lime is commonly linkedto the rate of dissolution of particles of slaked lime or calciumhydroxide, which increases as the outer surface of the particlesincreases and the size of the particles thus decreases. By virtue of thesame logic, the viscosity increases along with increasing solids contentand decreasing particle size. This is generally explained by the factthat the particles content per unit of volume increases. Therefore, anincreasing number of particles are close to each other, and thusinteract with each other and with the water. Such interactions areconsidered by some authors as being mainly of an attractive nature dueto the low surface charge of the slaked lime particles and their abilityto form hydrogen bonds with water molecules. These interactions thenlead to an increase in cohesion within the suspension and therefore toan increase in viscosity.

In the context of the present invention, it is necessary to distinguishthe reactivity of milk of lime which represents the rate of dissolutionof particles of hydrated lime from the reactivity of quicklime, that isto say the rate of reaction of quicklime with water to produce slakedlime and thus for example a milk of lime.

In fact, the reactivity of quicklime is generally characterised andmeasured by the procedure disclosed in the European standard EN459-2 andoften quantified by t₆₀, the time required for a volume of 600 cm³ ofwater initially at 20° C. to reach 60° C., on the basis of the additionof 150 g of quicklime.

With respect to the reactivity of milk of lime it is characterised forthe purposes of the present invention according to the work of vanEekeren et al. disclosed in “‘Improved milk-of-lime for softening ofdrinking water’, M W M van Eekeren, J A M van Paassen, C W A M Merks,KIWA NV Research and Consultancy, Nieuwegein, September 1993” producedand distributed by KIWA, the Royal Netherlands Water Research Institute(KIWA NV Research and Consultancy, Groningenhaven 7, PO Box 1072, 3430BBNieuwegein.)

For the purposes of this present invention, the procedure described hasbeen refined in a manner so as to improve the accuracy andreproducibility of results and has been tested on a multitude ofdifferent formulations of milk of lime.

According to the present invention, the rate of dissolution of theslaked lime in deionised water is measured in the form of an increase inthe electrical conductivity of the solution, under conditions in whichthe solution remains below the saturation point with respect to calciumhydroxide. In order to ensure that it remains below the saturationpoint, 0.1 g of hydrated lime is added to 700 g of water at 25° C.,which enables the solution to remain well below the solubility point ofcalcium hydroxide which is about 1.5 g per liter of solution at 25° C.(see for example the document “Solubilities of Inorganic & MetalorganicCompounds—Volume 2”, A Seidell, W F Linke, 1953, van Nostrand (publ.),p. 631).

To achieve this accurate and reproducible measurement, 500 g ofsuspension containing 2% by weight of slaked lime to be characterised isprepared, that is 10 g of slaked lime in 490 g of water. This suspensionand 700 g of deionised water are thermostatted at exactly 25° C. Aconductivity cell with a response time of 0.05 sec or less is used torecord automatically, by means of a data logger, the conductivity of thesample of 700 g of deionised water which is stirred vigorouslythroughout the measuring process, for example at a speed of 450rotations per minute (rpm) with a propeller rod stirrer having adiameter of 30 mm.

At the beginning of the measurement, 5 cm³ of the suspension of 500 gare injected into the sample of 700 g of deionised water and theconductivity value is recorded over time until the latter remains stablehaving thus reached a maximum value. The time to reach this maximumconductivity since the beginning of the measurement is denoted by t₁₀₀.In a similar manner too is defined as the time to reach 90% of themaximum conductivity. It is this obtained value t₉₀ which is consideredto represent the reactivity of milk of lime. More details on theprocedure for measuring the reactivity of milk of lime are available insection § 6.11. “Determination of the solubility index by conductivity”of the standard EN 12485: 2010.

Therefore according to the present invention, it is considered that thereactivity of milks of lime will be high if the magnitude of t₉₀ is lessthan or equal to 10 sec.

The document entitled “Studies on the particle properties of suspendedhydrated limes’, U. Wittneben, Zement-Kalk-Gips, Edition B, Vol. 33(10), p. 526-534, 1980” by Wittneben et al of the Research Associationof the German Lime Industry (Forschungsgemeinschaft des Verbands derDeutschen Kalkindustrie) also discloses that the differences in theshape of particles, in the size of particles and in the distribution ofparticle sizes play a significant role in determining the viscosity ofthe milk of lime. This is further confirmed by the recent work ofRodriguez-Navarro et al (“Microstructure and rheology of lime putty”’, ERuiz-Agudo and C Rodriguez-Navarro, Langmuir: the ACS journal ofsurfaces and colloids, Vol 26 (6), pp. 3868-3877, 2010). In thisdocument, in FIG. 4a , which is to be reviewed along with FIG. 4b , aprofile of particle sizes obtained by TEM (Transmission ElectronMicroscopy) digital image analysis can be seen that thus necessarilyonly reveals particles sizes between 0.01 μm and 1 μm, the remainder ofthe particles is illustrated in FIG. 4b and shows a multimodal profile.These authors have studied the suspensions produced from lime fromdifferent origins and have concluded that there is a link between thereactivity of the quicklime and the size, stability, and also theinteraction of the particles of slaked lime in suspension.

The authors were also able to correlate the influence of the shape,stability and interactions of particles on the viscosity of thesuspension, but also on the flocculation and agglomeration behaviour ofthe slaked lime particles of the suspension or of the milk of lime.

Since the agglomeration and the formation of flocs from slaked limeparticles alter the apparent particle size and distribution of the sizeof slaked lime particles, it is a logical conclusion that theagglomeration and the formation of flocs from slaked lime particles alsoalter the external surface of the particles, which is itself linked tothe reactivity of the milk of lime.

In conclusion, the milk of lime produced by limes of various origins andhaving varied reactivity values would commonly display, even underidentical conditions of slaking, rheological properties and reactionproperties that are different. Moreover, since the reactivity ofquicklime is generally dependent on the type of oven used for thecalcination of limestone into quicklime, the type of oven would alsoplay a role in determining the properties of the milk of lime, inaddition to the influence of the origin of the limestone.

Thus substitution of the quicklime from a specific production sitehaving characteristics that result from a particular combination of theorigin of the limestone, the type of oven used and the reactivity of thequicklime with the quicklime from another specific production sitehaving characteristics resulting from another combination including adifferent origin of limestone, another type of oven used and anotherreactivity of quicklime could lead to significant changes in theproperties of the milk of lime which would result therefrom. This wouldnecessarily have consequences with respect to the subsequent industrialapplication of this milk of lime.

In other words, the use of milk of lime for a particular applicationdoes not only entail choosing a feasible compromise between anacceptable viscosity and the required reactivity of milk of lime, butalso signifies that the conditions applied so as to produce such asuspension of lime could only be applied to a limited number or possiblyto a single source of commercially available lime. In extreme cases,this would mean that only one single source would be technicallypossible and economically feasible for producing a milk of lime havingspecific viscosity properties and a predetermined rate of reaction.

There is therefore a real need to develop milks of lime whose propertieswould be optimal and consistent, regardless of the sources of limestoneand burning method used.

With regard to this subject, milks of lime have been disclosed in theliterature and have properties that are optimised for specificapplications.

The documents WO 96/23728, DE 2714858, DE 4447321 A1, JP 2007/031212 A,CN 201316654 Y, SU 1186248 A, EP 1039964 B1 and SE 870408 disclosemethods for processing of milks of lime in which the properties of thesesuspensions are optimised for various applications by deagglomerationand even grinding of the milk of lime—which makes it possible to obtainreduced particle sizes—by different types of equipment that at times maybe very expensive.

Even though these documents teach ways to effectively increase thereactivity of the milks of lime, the result thereof is nevertheless anegative impact on the viscosity of the milks of lime thus obtained. Inmany cases, moreover, the authors compensate this increase in viscosityby a dilution in order to reduce the solids content, which is by nomeans desirable for many applications in that increasing amounts ofwater are therefore inevitably introduced, which leads to levels ofdilution that are too high for numerous applications.

Moreover, these documents pertaining to the state of the art do not takeinto account the influence of the origin of limestone, of the type ofoven used and the reactivity of the hydrated lime obtained and thereforedo not mention any solutions to compensate for variations related tothese influences.

For example according to the document DE 27148858, at the time of theindustrial exploitation, it had been demonstrated that the proposedmethod and equipment thereof functioned in a reliable manner and as aresult provided an appropriate quality of milk of lime only if powderedquicklime of ultra high quality, and ultra high purity and having ahomogeneous reactivity was used, thereby greatly complicating theoperation of the process disclosed.

Other limes of ultra high purity and very good quality have beendescribed and used to produce highly reactive milks of lime with fineparticles. According to the document CN 201316654 or the document EP1039965, the slaking of high purity lime is carried out in a ball mill.Unfortunately, these technologies require the use of a source of verypure quicklime, thus impeding the production of a product with broadindustrial applicability on account of high costs and limitedavailability.

According to the documents SU 1186248, JP 2007/031212 and SE 870408, ahigh purity quicklime is used in order to produce a suspension of ultrafine slaked lime particles. In the method described, a step of screeningby size is introduced so as to eliminate the inert material from thesuspension of slaked lime particles obtained. Unfortunately, thesetechniques are highly dependent on the reactivity of the quicklime andpresent the major disadvantage of requiring screening or sifting of asuspension of quicklime particles of very high viscosity composed offine particles.

The document JP 2007/031212 discloses another known technique forcompensating for the negative effect of the increase in viscosity,namely the use of dispersant additives, which act on the repulsionbetween the slaked lime particles and in this way diminish the viscosityof the resulting suspension (see also document US2004/0258612).

Unfortunately, these additives are in many cases undesirable for the enduser as they result in the addition of an “impurity” in the milk of limeand therefore in the process of the end user or in the finished product,not to mention that sometimes an active interference between thedispersant and the process or the final application has to be reckonedwith, mainly because of the nature of the active surface of the majorityof these additives.

It is also a known practice to reduce the specific surface area of theparticles so as to reduce the viscosity (see document WO2005/014483).

The invention aims to overcome the drawbacks of the state of the art byproviding an ultra fine milk of lime having high reactivity and lowviscosity which can use any source of quicklime, without resorting tothe use of special additives such as dispersants.

In order to solve this problem, the aim is to provide, according to theinvention, a high reactivity composition of milk of lime as indicated atthe beginning characterised in that said particles of slaked lime arecomposed of slaked lime particles that have a particle size described bya particle size distribution profile that is narrow and monomodal.

Within the context of the present invention the terms “narrowdistribution profile” or “narrow particle size distribution profile” areunderstood to mean that the difference between d₉₀ and d₁₀ is less thanor equal to 15 microns, preferably less than or equal to 10 microns. Thenotation d_(x) with 0<x<100 represents a diameter, expressed in microns,relative to which x % of the particles are smaller.

The product according to the present invention is thus based on an ultrafine milk of lime, whose particle size distribution profile is narrowand monomodal (where the width of the distribution profile at d₉₀−d₁₀<15μm), that is to say, which has a single narrow peak corresponding to asingle family of sizes of slaked lime particles. The slaked limeparticles therefore have a high reactivity (rate of dissolution) becauseof their fineness while having a reaction behaviour that is homogeneousbecause of the homogeneity of the size of particles obtained.

Indeed, conventional milks of lime typically have a particle sizedistribution profile which is multimodal (having multiple peaks) andtherefore include several populations of slaked lime particles. Thesedifferent populations of particles would typically be composed of finefractions having increased reactivity and coarser fractions havingreduced reactivity. In addition other intermediate fractions may bepresent, depending on the modes of the particle size distributionprofile observed. These intermediate fractions react according todifferent rates of reaction. This distribution of reactive behaviour dueto various rates of reaction results in a reaction behaviour(reactivity) of the milk of lime that is non homogeneous, which isproblematic for numerous applications, as mentioned here above.Applications that are dependent on the rate of dissolution of the milkof lime include by way of example, however without any limitation, therapid neutralisation processes, especially during the production ofprecipitated calcium carbonate—PCC, processes of synthesis of chemicalproducts such as ethylene oxide or propylene oxide or even flue gastreatment processes.

The high reactivity milk of lime having a particle size distributionprofile that is narrow and monomodal according to the present inventiondoes not have any distribution of the rate of reaction. On the contrary,the milk of lime composition according to the present invention reactsin a homogenous manner because of the homogeneous distribution of theslaked lime particles in the reaction medium, which is due to the narrowand monomodal particle size distribution profile and therefore due tothe homogeneity of the size of particles.

For example, it is known that in the production of PCC, particles ofCa(OH)₂ having a particle size distribution profile that is broaderleads to a particle size distribution profile for the PCC that is alsobroader, which is by no means desirable since one of the advantages ofPCC relative to ground calcium carbonate lies precisely in a narrowparticle size distribution.

A milk of lime according to the invention having a narrow and monomodalparticle size distribution profile makes it possible to achieveprecisely this narrow particle size distribution profile in the PCCproduction and thereby improves the quality thereof.

Another example can be found in the use of milk of lime in theproduction of propylene oxide, which perfectly illustrates theimportance of lime with high and homogeneous reactivity as a reagent inchemical reactions. Propylene oxide is synthesised on an industrialbasis primarily for the production of epoxy resins. During the steps ofsynthesis, dichlorohydrin (ClH₂C—CHOH—CH₂Cl) reacts with a hydroxide(sodium hydroxide, or preferably calcium hydroxide) in epichlorohydrin(H₃C₂O—CH₂Cl), to generate the desired epoxy group (C₂O).

This reaction must be rapid since epichlorohydrin has the tendency toreact with water from the reaction medium to form monochlorohydrin(CH₂OH—CHOH—CH₂Cl) and then glycerol (CH₂OH—CHOH—CH₂OH), which destroysthe epoxy groups. It is therefore essential in this type of applicationto be able to avail of high reactivity milk of lime in order to achievehigh reaction yields, and to be able to extract the epichlorohydrinformed from the reaction medium as quickly as possible. This can only becarried out in a subsequent step of the process, meaning that thewithdrawal of epichlorohydrin may be performed only when the reaction ofepichlorohydrin formation has ended, so as to avoid the presence ofdichlorohydrin which has not reacted. Indeed, the presence ofdichlorohydrin remaining unreacted would be detrimental to the overallperformance and yield of the reaction relating to epichlorohydrinformation and would remain in the form of impurities within the process.

The milk of lime according to the present invention is therefore anideal candidate for the process of production of epichlorohydrin, inthat it has a rate of reaction that is very high due to its ultrafineness and is homogeneous, due to its narrow and monomodal particlesize distribution profile.

Advantageously, the particles have a particle size d₉₈<10 μm (microns)and/or a particle size d₅₀<1.5 μm and/or a particle size d₃₀≤1 μm asmeasured by sedimentation (for example with a Micromeritics Sedigraphapparatus).

Alternatively, said particles have a particle size d₉₈ less than orequal to 10 μm and/or a particle size d₅₀ less than or equal to 3 μm,preferably less than or equal to 2.5 μm, in particular less than orequal to 2 μm, and/or a particle size d₁₀ less than or equal to 1 μm,preferably d₂₅ less than or equal to 1.5 μm, in particular less than orequal to 1 μm, as measured by laser diffraction.

As can be seen from the values of particle sizes given here above, theparticles are very fine, in addition to presenting a narrow andmonomodal particle size distribution profile, which is contrary to theteaching of document EP0943590.

Advantageously, the size d¹⁰ of the particles is greater than or equalto 0.1 μm, as measured by laser diffraction, and this is so, unlikeconventional milks of lime that do not have this fineness. In aparticular embodiment, the composition of slaked lime according to thepresent invention has a viscosity less than 350 mPa·s (millipascalsecond), preferably less than 250 mPa s, and in a more preferentialmanner less than 200 mPa s, in an advantageous fashion less than 100 mPas as measured by a standard Brookfield DV-III Rheometer equipped with a3 or 63 spindle, at a speed of rotation of 100 rotations per minute(rpm).

Preferably, the composition is a suspension of slaked lime in the formof milk of lime having a solids content greater than or equal to 2%,advantageously greater than or equal to 5%, in a preferential mannergreater than or equal to 10%, in particular greater than or equal to12%, in a particularly preferential manner greater than or equal to 15%,relative to the total weight of the suspension. In general, thecomposition according to the invention is a suspension (slurry) ofslaked lime having a solids content less than or equal to 30%, inparticular less than or equal to 25%.

In a particularly advantageous embodiment according to the invention,the slaked lime particles have a rate of dissolution in distilled water,as measured by the KIWA procedure such that 90% of the particles ofslaked lime are dissolved in less than 10 seconds, in particular in lessthan 8 seconds, preferably in less than 5 seconds, and in a morepreferential manner in less than 3 seconds.

Other embodiments of the composition of slaked lime according to theinvention are indicated in the appended claims.

The invention also relates to a method for the production of highreactivity milk of lime, comprising of the following consecutive stepsof:

-   -   a) the slaking of quicklime by applying a proportion by mass of        quicklime relative to water greater than 1 to 8, in particular 1        to 6, and less than 1 to 3 so as to form a lime suspension,    -   b) particle size cutting of said lime suspension, possibly        diluted, with the obtaining of at least one coarse first        fraction to be ground and one fine second fraction    -   c) particle size reduction by wet ball grinding of said at least        one coarse first fraction to be ground and    -   d) obtaining of said milk of lime with high reactivity, possibly        after dilution.

The method according to the present invention has the great advantage ofallowing the use of any type of industrial lime having an altogetherstandard reactivity and chemical properties, without the need forresorting to a base of high purity lime, in order to produce asuspension of lime which may be granulometrically cut with ease. Thestep of particle size cutting or screening eliminates the inert materialas well as any type of particles that are too large in size, such ascalcium carbonate in the unfired form or all other impurities such assilica, for example. The wet ball grinding of the said at least onecoarse first fraction to be ground, resulting from the step of particlesize cutting, leads to a product in the form of a slaked lime suspensionwhose constituent particles are very fine and in a very reliable mannerpresent a narrow and monomodal particle size distribution profile andwhich can be controlled.

Moreover, the optimisation of the process according to the inventionmakes it possible to reduce the increase in viscosity originating fromthe increase in fineness of the particles in order to improve thereactivity of the milk of lime according to the invention.

The method according to the present invention thus makes it possible toproduce a milk of lime that is superior to conventional milks of lime interms of rate of reaction and reaction behaviour, for a wide range ofapplications as mentioned here above, since it has a narrow andmonomodal particle size distribution profile.

The term “monomodal” in this context signifies that the measureddifferential distribution of the size of particles obtained byconventional measurement methods described here above shows only onesingle mode or peak, preferably distributed evenly around the value ofd₅₀.

Conventional slaked lime, which is a nanocrystalline material, comprisesaggregates and agglomerates of nanocrystals of calcium hydroxide of 20to 120 nm and therefore typically presents a particle size distributionprofile that is broad and multimodal. Reduction of the size of particlesof this conventional slaked lime indeed requires more than just thesimple grinding or simple conventional particle size reduction in orderto obtain a monomodal and narrow distribution.

In fact, the release of individual nanocrystals or submicron aggregatesduring the slaking, particle size cutting and granulometric reductionshould be kept as low as possible given that, as such, these finenanoscale particles have a tendency to significantly increase theviscosity.

Moreover, some of these nanoparticles are crystallographically unstableand recrystallize, which changes the viscosity by increasing it ordecreasing it, which is obviously not desirable because these changes inthe rheological properties of the suspension are neither controllablenor predictable.

As it is known by the person skilled in the art, during the wet grindingin a ball mill, the particles of the material being ground are brokendue to the energy transferred by the balls to the slaked lime particlesduring collisions, which moreover also depends on the mass of the balls,typically 1.5 to 2.5 kg of balls per cubic decimeter (dm³) of mill, andon their speed related to the stirring (speed of rotation of the mill).This transferred energy must exceed the minimum breaking energy of theparticles, in order to induce the breaking of the particles of slakedlime. The rate of stirring and energy depend on the volume of the wetmill. For example, for a mill of 100 dm³, the rate of stirring is of theorder of 650 rpm, with 55 to 75 kW of power; for a mill of 0.5 m³, therate of stirring is about 350 rpm and with 160 to 200 kW of power. Atthe same time, it is necessary to avoid inducing excessive energy whichwould lead to excessive fragmentation and to the equally unsuitablegeneration of submicron fragments.

This minimum breaking energy is commonly a property of the material, butin the case of slaked lime it also depends on the source of the lime, onthe conditions of slaking and the fractionation of the size ofparticles. These criteria are thus used to optimise the grinding, whichenables the production of a milk of lime for which the particle sizedistribution profile is narrow and monomodal. Quite certainly, to knowthe rate of submicron particles is also considered a critical factor.

Advantageously, the said particle size reduction (granulometricreduction) by ball grinding is carried out in a ball mill containingballs of oxide or zirconium silicate or glass for grinding which areless than 1.4 mm in size.

In a particular embodiment, said particle size cutting comprises ascreening by size on a vibrating screen with mesh size of 250 μm,preferably with mesh size less than or equal to 100 μm and wherein saidcoarse first fraction to be ground is the fraction passing through thescreen with mesh size of 250 μm, preferably with mesh size less than orequal to 100 μm and said fine second fraction is a fraction of rejectedresidue essentially consisting of inert particles or impurities to beremoved.

In a variant, said particle size cutting comprises a double screening bysize on a first vibrating screen with mesh size of 250 μm, and on asecond vibrating screen with mesh size less than or equal to 100 μm,with the obtaining of a coarse first fraction to be ground, of a finesecond fraction and a third fraction, wherein said coarse first fractionto be ground is the fraction passing through the meshes of the 250 μmscreen and rejected as residue at the screen with mesh size less than orequal to 100 μm, said fine second fraction is a fraction of residuerejected at the 250 μm mesh screen, essentially consisting of inertparticles or impurities to be removed and said third fraction is afraction passing through the meshes of the screen with mesh size lessthan or equal to 100 μm. This third fraction is generally to be ground.

In a variant according to the present invention, the said particle sizecutting is carried out by hydrocycloning of said lime suspension,possibly diluted, with the obtaining of a fraction of particles whoseparticle size is less than 10 μm as the second fraction and a fractionof particles having a particle size greater than 10 μm as the coarsefirst fraction to be ground and wherein said milk of lime having highreactivity to water is obtained by mixing of said fine second fractionand said ground coarse first fraction, possibly after dilution.

In yet another alternative embodiment according to the presentinvention, the particle size cutting comprises a first step of screeningby size on a vibrating screen with mesh size of 250 μm, preferably withmesh size less than or equal to 100 μm, making it possible to obtain anaccepted fraction passing through the meshes of the screen with 250 μmmesh size, preferably with mesh size less than or equal to 100 μm and afraction of rejected residue essentially consisting of inert particlesor impurities to be removed, and a second step of hydrocycloning of saidaccepted fraction, possibly diluted, with the obtaining of a fraction ofparticles whose particle size is less than 10 μm as the second fractionand a faction of particles having a particle size greater than 10 μm asthe coarse first fraction to be ground and wherein said milk of limehaving high reactivity to water is obtained by mixing of said finesecond fraction and said ground coarse first fraction, possibly afterdilution.

In a variant, according to the invention, the said particle size cuttingcomprises a first step of double screening by size on a vibrating screenwith mesh size of 250 μm, and on a vibrating screen with mesh size lessthan or equal to 100 μm, with the obtaining of an accepted fractionpassing through the meshes of the screen with 250 μm mesh size butrejected as residue at the vibrating screen with mesh size less than orequal to 100 μm, of a fraction of residue rejected at the vibratingscreen with 250 μm mesh size, essentially consisting of inert particlesor impurities to be removed, and of a fraction accepted at the vibratingscreen with mesh size less than or equal to 100 μm, as well as a secondstep of hydrocycloning of at least one of the accepted fractions,possibly diluted, with the obtaining of at least one fraction ofparticles whose particle size is less than 10 μm as the fine secondfraction and at least one fraction of particles whose particle size isgreater than 10 μm as the at least one coarse first fraction to beground and wherein said milk of lime having high reactivity to water isobtained by mixing of said fine second fraction and the at least oneground coarse first fraction, possibly after dilution.

As it may be noted, said step of particle size cutting can be carriedout in different ways by selecting one or more steps of particle sizecutting in the form of vibrating screen or hydrocycloning processes. Thevibrating screen with 250 μm mesh size can be coupled with ahydrocyclone or with one or more other screens with lower mesh sizessuch as, for example, mesh sized 10 μm or less, possibly also coupledwith a hydrocyclone.

More particularly, in the method according to the present invention,said at least one ground coarse first fraction and said fine secondfraction are mixed in a proportion of between 20% and 75% for the firstcoarse fraction and 25% and 80% for the second fraction, possibly afterdilution.

Advantageously said quicklime is a quicklime having a reactivity tenmeasured according to the reactivity test described in the standardEN459-2 within a range of 0.5 to 20 minutes, preferably between 1 to 10minutes, preferably less than 5 minutes.

In an advantageous embodiment, the slaking is carried out with water,for example process water at a temperature of about 40° C.

In a particularly advantageous embodiment of the method according to thepresent invention, said slaking is carried out in a lime paste slaker.

Other embodiments of the method according to the invention are set forthin the appended claims.

Other characteristic features, details and advantages of the inventionwill become apparent from the description given here below, withoutlimitation and with reference being made to the accompanying drawings.

FIG. 1 is a graph representing the distribution of sizes of particlesthat is narrow and monomodal identified by a set of characteristicdiameters for the milk of lime according to the invention depending onthe grinding energy applied.

FIG. 2 is a graph representing the measured viscosities of suspensionsof milk of lime according to the invention having various grades offineness, produced under different conditions of grinding (grindingenergy applied).

FIG. 3 is a graph representing an example of monomodal distribution ofparticles of a milk of lime, as measured by laser diffraction.

In the figures, identical or similar elements have the same identifyingreferences.

The present invention therefore relates to a method of producing asuspension of milk of lime in three steps that make it possible toproduce an ultra fine milk of lime with high reactivity wherein theparticle size distribution profile is narrow and monomodal.

The first step consists of slaking the quicklime in a paste slaker inorder to produce an aqueous suspension of lime, the second step consistsof an operation of particle size cutting also known as particle sizescreening which makes it possible amongst other things, to eliminate theinert particles and impurities such as silica or limestone from the limesuspension, possibly diluted. The third step is a step of wet ballmilling which makes it possible to obtain a particle size distributionprofile that is narrow and monomodal along with a reduced inertparticles content.

This particular narrow and monomodal particle size distribution makes itpossible to obtain an ultra fine milk of lime that is homogeneous interms of particle sizes and has high reactivity that is alsohomogeneous.

In one embodiment according to the present invention, the acceptedfraction of milk of lime output from the 250 μm vibrating screen isprocessed in a second step of particle size cutting like for example ina hydrocyclone or with one or more vibrating screen/s having smallermesh size, in order to obtain a fraction the particle size of which isalready as expected and at least one coarser fraction to be ground.

Each coarser fraction or one or more of them are then ground accordingto the present invention in a step of wet grinding in a ball mill inorder to form particles of slaked lime having a narrow and monomodalparticle size distribution profile that is well defined in the contextof the present invention. The accepted fine fraction, the particle sizeof which is already as expected and at least one of the coarser groundfractions are then mixed or used separately.

In one variant according to the invention, the quicklime is slaked in astirred reactor, in a ball mill or in a high shear stress slaker,instead of a lime paste slaker, provided that the slaked lime particlesare not too small in size, which would hamper any further particle sizecutting to eliminate the inert fraction, on account of the possibleclogging of the screen caused.

As it may be noted, the choice of conditions for slaking and particlesize cutting depends largely on the characteristics of the milk of limeresulting from the source of quicklime. The object of the invention isto produce, regardless of the characteristics or qualities of the sourceof lime, a composition having a milk of lime base, of which the particlesize distribution profile is narrow and monomodal. The flexibilityachieved by the method according to the invention with respect to thesource of lime is not the only advantage. In fact, the method alsoallows for the use of different sources of slaking water unlike in theprevious documents where water had also to be of high purity. Inaddition, the loss during screening is reduced, which offers significanteconomic benefits.

The milk of lime according to the present invention, having said narrowand monomodal particle size distribution profile enables the use thereofin applications with short processing periods such as rapidneutralisation processes, synthesis of chemical products consumingslaked lime, rapid softening of water or mineralisation of the latter oreven the precipitation of calcium carbonates.

The composition of milk of lime according to the present invention isthus characterised by the fact that said particles advantageously have aparticle size d₉₈<10 μm, a particle size d₅₀<1.5 μm and a particle sized₃₀<1 μm as measured by sedimentation (for example with a MicromeriticsSedigraph apparatus), and/or a particle size d₉₈ less than or equal to10 μm, a particle size d₅₀ less than or equal to 3 μm, preferably lessthan or equal to 2.5 μm, in particular less than or equal to 2 μm and aparticle size d₁₀ less than or equal to 1 μm, preferably d₂₅ less thanor equal to 1.5 μm, preferably less than or equal to 1 μm, as measuredby laser diffraction by means of using a device of the typeBeckmann-Coulter LS 13 320 or Horiba LA950.

The viscosity of the milk of lime according to the present invention isless than 350 mPa s, preferably less than 250 mPa s, and in a morepreferential manner less than 200 mPa s, in an advantageous manner lessthan 100 mPa s, as measured by the standard Brookfield Rheometer DV-IIIwith a speed of rotation of 100 revolutions per minute.

The solids content of the milk of lime according to the presentinvention is in addition greater than or equal to 2%, advantageouslygreater than or equal to 5%, in a preferential manner greater than orequal to 10%, in particular greater than or equal to 12%, in aparticularly preferential manner greater than or equal to 15%, relativeto the total weight of the suspension. In general, the compositionaccording to the invention is a suspension of slaked lime having asolids content less than or equal to 30%, in particular less than orequal to 25%.

Finally, the rate of dissolution in distilled water of the lime milkaccording to the present invention, as measured by the KIM procedure, isadvantageously such that 90% of the particles of slaked lime aredissolved in less than 8 seconds, preferably in less than 5 seconds andin a more preferential manner in less than 3 seconds.

EXAMPLES Example 1

3 samples of lime B, D, E, from three different geographical sources(respectively north of France, Portugal and central France) having abehaviour with respect to formation of milk of lime that is similarduring slaking are slaked in a vertical hydrator measuring 15 dm³equipped with a propeller rod stirrer, that is 70 mm in diameter, byadding hot water at a temperature of 40° C. to quicklime, based on alime/water proportion of ⅕ by weight. The processing time period is 30minutes with a speed of rotation of 400 rpm. The milks of lime thusobtained are screened on a vibrating sieve with mesh size of 90 μm. Eachfraction accepted is diluted until a suspension of 15% by weight ofsolid material is obtained and is wet ground in a ball mill filled up to85% with glass balls that are 0.8 mm to 1.2 mm in size, at a speed ofrotation of 2200 rpm for a period of about 2.5 min until the obtainingof a d₉₈<10 μm and a d₅₀<3 μm as measured by laser diffraction.

The suspensions obtained have a particle size distribution profile thatis narrow and monomodal and a viscosity less than 350 mPa s as mentionedin the table.

Comparative Example 1

Two samples of lime, A and C, from two geographically different sources(southern Poland and south of France) having a behaviour with respect toformation of milk of lime that is slightly different compared to thethree limes of Example 1 are hydrated, screened and ground under thesame conditions as in Example 1. Before grinding, suspension A has aviscosity that is significantly higher than that of suspensions B, D andE. For suspension C, the viscosity is much higher. After grinding,suspension A has a moderately higher viscosity and suspension C is fartoo viscous for proper industrial handling (see table). In both cases,the high viscosity of the suspension can be avoided by adjusting theconditions of slaking in particular by reducing the temperature of theslaking water by around 15® C and/or by decreasing the stirring by about200 rpm, in order to obtain particles the breaking energy of which ishigher or by using grinding conditions which provide less energy percollision (see Example 2).

Example 2

Lime samples A and C are hydrated and screened under the same conditionsas in Example 1, but ground at 2100 rpm with finer balls that arebetween 0.5 mm and 0.7 mm in size, that is to say, which transmit about60% lower energy by collision. Even though the fraction of ungroundslaked lime (A1 and C1) is similar to that in Comparative Example 1, anddespite the fact that the ground slaked lime (A1 and C1) is finer thanthe ground lime in Comparative Example 1 (A and C), the viscosity of thesuspension C1 is significantly reduced (see table) and the viscosity ofthe suspension A1 is not very strongly increased, in spite of theextreme fineness of the milk of lime obtained having a d₉₈ of about 5μm, a d₅₀ of about 1.5 μm and d₂₅ of about 1 μm.

TABLE analytical results pertaining to the suspensions in Examples 1 and2 and in Comparative Example 1. The distribution of particle size wasmeasured by means of the device Beckmann-Coulter Laser DiffractionParticle Sizer LS 13 320 Viscosity KIWA Sample % Solid (mPa · s) T₁₀₀(s) T₉₀ (s) d₉₈ (μm) d₉₅ (μm) d₉₀ (μm) d₅₀ (μm) d₂₅ (μm) Distribution ofparticle sizes of a milk of lime that is screened but not ground A 15.1140 65.4 9.4 61.0 42.2 28.3 5.7 2.5 B 15.1 10 92.5 20.9 78.2 58.0 39.07.3 3.1 C 15.2 350 55.3 9.6 61.9 46.6 30.9 5.4 2.4 D 15.2 45 100.5 17.480.1 57.9 33.4 5.6 2.7 E 15.1 21 64.7 15.5 73.6 55.6 35.4 6.4 3.1 A115.0 140 135.4 5.0 62.1 46.2 27.9 4.7 2.2 C1 14.8 410 49.5 4.6 59.2 42.028.4 4.8 2.0 Distribution of particle sizes of a milk of lime that isground A 15.3 390 1.8 1.6 8.9 7.7 6.5 2.3 1.4 B 14.9 190 4.2 3.0 9.1 7.96.6 2.4 1.4 C 15.2 1760 4.8 2.8 8.9 7.7 6.4 2.2 1.3 D 14.9 265 13.7 0.89.2 8.0 6.7 2.4 1.4 E 14.8 125 5.4 4.0 8.7 7.6 6.5 2.5 1.5 A1 15.0 5305.6 4.6 6.1 5.1 4.1 1.6 1.0 C1 14.7 660 5.0 4.6 4.9 4.0 3.0 1.4 0.9

Example 3

In order to illustrate the influence of the conditions of slaking on thegrinding of milk of lime and the properties of the ultra fine milk oflime thus obtained, another sample (B1) from the same geographicalsource as the sample B is hydrated under two different sets ofconditions.

In the first case, one portion of the sample is hydrated by adding hotwater at a temperature of 40° C. to the quicklime based on a lime/waterproportion of ⅕ by weight.

In the second case, another portion of the sample is hydrated byaddition of warm water at a temperature of 20° C. based on the samelime/water proportion of ⅕ by weight. The milks of lime thus obtainedare then screened on a vibrating screen with mesh size 250 μm and theaccepted fractions are then sufficiently diluted so as to obtain a solidcontent of 13% by weight and divided into different sub samples. Thesesub samples are then ground under different conditions in a ball mill,in particular by varying the rotation speed of the mill and theprocessing time period, but generally otherwise, under the same overallconditions as in Example 1, that is to say, with the same size of balls,the same kind of balls, the same fill rate, etc.

The distribution of particle sizes obtained is presented in FIG. 1. Themeasured viscosities of the suspensions obtained are shown in FIG. 2.

In an expected manner, the characteristic diameters decrease when thegrinding energy applied increases. However, slaking with water having atemperature of 20° C. results in a fraction of milk of lime havingparticles which are coarser, that is to say having higher particle sizesd₉₀, d₉₅ and d₉₈. The fractions of milk of lime with larger particleslead to a milk of lime that is coarser post grinding by applying thesame grinding energy in comparison with milk of lime hydrated with waterhaving a temperature of 40° C.

While the suspension slaked by addition of water having a temperature of40° C. may be ground under the conditions presented in FIG. 1 in orderto achieve the particle size distribution profile in accordance with thepresent invention, this was not the case for the milk of lime obtainedwith water having an initial temperature of 20° C. The milk of limeslaked with water having a temperature of 20° C. is a bit too coarse interms of d₉₈.

Surprisingly, the milk of lime obtained with water initially at atemperature of 20° C. has a higher viscosity in spite of the presence ofa fraction of particles of larger size in this milk of lime as comparedto that obtained with the slaking water initially having a temperatureof 40° C.

In both these two cases, the viscosity increases with the grindingenergy applied, that is to say, when the particle size decreases for therespective milks of lime.

It may therefore be concluded that the conditions for slaking, screeningor particle size cutting (coarse fraction before grinding) and theconditions for grinding may be optimised in relation to each other inorder to produce an improved milk of lime with higher reactivity andreduced viscosity.

It is indeed understood that the present invention is in no way limitedto the embodiments described here above and that modifications may wellbe made thereto without departing from the scope of the appended claims.

The invention claimed is:
 1. A composition of high reactivity milk oflime comprising particles of slaked lime in suspension in an aqueousphase, characterized in that said particles of slaked lime are composedof slaked lime particles that have a particle size described by aparticle size distribution profile that is narrow and monomodal; thecomposition having a viscosity less than 350 mPa·s (millipascal second),as measured by a standard Brookfield DV-III Rheometer at a speed ofrotation of 100 rotations per minute (rpm).
 2. A composition accordingto claim 1, wherein said particles have a particle size d₉₈ less than orequal to 10 μm, a particle size d₅₀ less than or equal to 1.5 μm, and aparticle size d₃₀ less than or equal to 1 μm, as measured bysedimentation.
 3. A composition according to claim 1, wherein saidparticles have a particle size d₉₈ less than or equal to 10 μm, aparticle size d₅₀ less than or equal to 3 μm, and a particle size d₁₀less than or equal to 1 μm, as measured by laser diffraction.
 4. Acomposition according to claim 1, wherein the composition is asuspension of slaked lime in the form of lime milk having a solidscontent greater than or equal to 2%, relative to the total weight of thesuspension.
 5. A composition according to claim 1, wherein thecomposition is a suspension of slaked lime in the form of lime milkhaving a solids content less than or equal to 30%.
 6. A compositionaccording to claim 1, having a rate of dissolution in distilled water,as measured by the KIWA procedure, wherein said rate of dissolution ismeasured in deionized water, in the form of an increase of electricconductivity of the solution, said electric conductivity being measureduntil said electric conductivity remains stable and reaches a maximalvalue, said electric conductivity being such that 90% of the particlesof slaked lime are dissolved in less than 8 seconds.